I suppose that if you have tried a lot of times, maybe you got your ip banned temporarily from the server(admins got nothing to do with this, since it is a defence against DoS attacks.). If it is not the case, you can try any of the "solutions" we have post in some other topics.

When you purchase cubits using your mobile phone, is the money deducted off your airtime or do you get billed on your contract?

If you have airtime on your phone, it will be deducted from it. If you do not, your contract will be billed (if you have that kind of contract). Please see Zong's website for detailed information about Zong payments.